1. Field
The present disclosure relates to a composite electric wire structure and a method for manufacturing the same.
2. Description of the Related Art
Graphene is a material having a planar (two-dimensional) structure wherein carbon atoms form a hexagonal lattice and exhibits different physical properties from graphite having a three-dimensional structure, carbon nanotubes having a one-dimensional structure, fullerene having a zero-dimensional structure, etc. As reported up to now, a single-layered graphene film exhibits unique characteristics distinguished from other carbon materials, with an electron mobility of about 150,000 cm2V−1S−1, an optical transparency of about 97.5% and a surface area of about 2600 m2g−1. In particular, because electrons behave in graphene as if they were massless due to the peculiar electronic structure of the graphene, electron transport in graphene is very fast. Particularly, it is reported that graphene exhibits electrical conductivity which is 100 times or higher than that of copper.
Meanwhile, at present, electric wires are made of metals, particularly copper (Cu) which is richer and relatively inexpensive as compared to other metals. However, a thin copper electric wire has the problem of power loss because dissipation of heat is difficult. And, a thick copper electric wire has the problem that electrical conductivity is decreased due to the skin effect by which electrons are distributed near the surface of the copper wire. In addition, metals are disadvantageous in that they are easily oxidized in the air and lose their inherent properties. Therefore, researches have been conducted to replace metal wires with flexible carbon materials having superior electrical, thermal and structural properties such as carbon nanotubes or graphene. However, although carbon nanotubes have high length-to-diameter ratios, the length is short to be used as electric wires. And, although carbon fibers have long length and mechanical properties suitable to be used as electric wires, they have low electrical conductivity.
Some researchers have conducted researches to replace metal electric wires with graphene only (Korean Patent Application Publication No. 2011-93666). According to this method, a metal layer is patterned to form a linear catalyst layer. After forming graphene in the catalyst layer via chemical vapor deposition, an electric wire formed of graphene fiber is obtained by removing the catalyst layer. There is another method of depositing a metal on a polymer fiber and forming graphene on the resulting metal layer via chemical vapor deposition (US Patent Application Publication Application No. 2013-0140058).
However, the inventors of the present disclosure have found out that the graphene electric wire is not uniform in the shape of the graphene and has a structural problem of short length similarly to the problem of the carbon nanotubes. In addition, the methods are inapplicable to a continuous process because they employ chemical vapor deposition and the fiber structure may be broken or short fibers may be formed during the step of removing the metal layer.
Also, there are methods where a polymer layer containing a carbon source is coated on the surface of a metal wire and then a graphene layer is formed by irradiating microwaves or intense pulsed light (IPL) (Korean Patent Application Publication No. 2013-58389, Korean Patent Application Publication No. 2013-51418) or where a graphene layer is formed on the surface of a metal wire via chemical vapor deposition (Current Applied Physics, K. J. Yoo et al., 2012, 12, 115-118).
According to the existing methods of forming a graphene layer on the surface of a metal wire described above, thick graphene layers are coated continuously on the surface of a metal wire as the whole surface of the metal wire is coated with the graphene layer.
FIGS. 1A and 1B schematically show the structure of graphene formed on a metal wire according to prior art. FIG. 1A schematically shows graphene coated on a metal wire according to the prior art, Korean Patent Application Publication No. 2013-58389 (FIG. 2 of Korean Patent Application Publication No. 2013-58389) and FIG. 1B shows the actual image of graphene coated on a metal wire according to the prior art, Korean Patent Application Publication No. 2013-51418 (FIG. 5 of Korean Patent Application Publication No. 2013-51418).
As can be seen from FIGS. 1A and 1B, graphene is formed on the whole surface of a metal wire in the Korean Patent Application Publication No. 2013-58389 (see FIG. 1A) and graphene is thickly coated on a metal wire in the Korean Patent Application Publication No. 2013-51418 (see FIG. 1B). The prior arts report that the coating of a graphene layer on the metal wire as such leads to improved electrical conductivity, etc.
However, the inventors of the present disclosure have found out that the existing method of forming a graphene layer on the metal wire surface as such has the following problems.
That is, a sheath such as an insulating sheath or a conductor shield is formed on the metal conductor wire for electrical insulation, and thus the sheath contacts with the conductor wire. When the graphene covers the whole surface of the metal wire as in the existing method, the graphene inevitably contacts the sheath such as the insulating sheath etc. However, because the graphene is a carbon material and exhibits poor adhesion to the sheath such as the insulating sheath or the shield layer which are made of an insulating polymer, the structure where the graphene is continuously coated on the metal wire results in a decrease of the adhesion of the sheath to the wire. As a result, during use of the electric wire, the sheath such as the insulating sheath etc. may burst, or short circuit may occur, or shielding property decreases due to the decreased adhesion or decreased insulation. These problems occur very frequently when the electric wire is bent, wound, etc. In particular, for high-capacity electric wires such as the power transmission cables used in transmission towers, short circuit may be very dangerous. Accordingly, the existing method of coating graphene on a metal wire involves a significant safety problem and may be limited in actual application. The existing method does not consider these problems.
In addition, when the graphene layer is coated on the metal wire surface, the graphene layer may be broken under the use environment of the electric wire, especially when the electric wire is bent or wound, graphene may be released from the metal wire as the graphene layer is broken, resulting in decreased electrical conductivity or mechanical properties such as flexibility, elasticity, etc.
Meanwhile, because the electric wire is manufactured by a continuous process, the manufacturing speed greatly affects the cost of the electric wire. Accordingly, a method for manufacturing a composite electric wire structure in commercially viable large scale wherein the composite electric wire structure may exhibit high safety without sheath bursting or short circuit under the use environment of the electric wire, especially when the electric wire is bent or wound, and as well without decrease in electrical conductivity or mechanical properties is also necessary.